The boiler system formed of a plurality of boilers each having a relatively small capacity exhibits a variety of advantages as compared with a system which employs a single boiler of a large capacity. To be specific, there is generally almost no chance for the large capacity boiler to be operated under a condition of the maximum output. It is therefore quite rare that its potential evaporation capability is effectively utilized. In sharp contrast with this, the boiler system based on combinations of small capacity boilers is arranged in such a way that in the great majority of cases the individual boilers function at their maximum output, and it follows that the potential capabilities thereof are efficiently exhibited. This is the most of the advantages mentioned above.
In general, the boiler is inevitably required to go through the preparatory stage which is commonly referred to as "prepurge" (ventilating or scavenging) and a subsequent low output state during a process originating from the halting state to a high output state (high combustion state), irrespective of the magnitude of capacity thereof. The preparatory stage causes not a little delay in time when starting the boilers. For this reason, it is desirable to remain the boilers which have once started in the continuous combustion state to the greatest possible degree, thereby causing the time lag at the starting time to be minimized, and making the boilers follow fluctuations in load (the amount of evaporation required per unit time) on the system quickly and smoothly.
For the purpose of satisfying the above-mentioned requirements, a wide variety of systems for controlling the boiler system have heretofore been proposed. Among conventional control systems, the arrangement of a well-known control system is such that the respective boiler units which are combined to constitute a boiler system are stepwise controlled at, e.g., two stages--i.e., these boiler units are shifted alternately between the high and low output states, or, that the boiler units are consecutively controlled to vary the outputs thereof with no interruption between the predetermined maximum and minimum values. A typical system in the former case is disclosed in, e.g., Japanese Patent Public Disclosure No. 81401/79, while a system in the latter case is disclosed in, e.g., U.S. Pat. No. 3,387,589.
In all these known systems for controlling the boiler system, a specific boiler unit among the plurality of boiler units, viz., the boiler unit which starts lastly, is operated to shift alternately from the low output state to the high output state in accordance with fluctuations in load, such a boiler unit is controlled to be brought into the halting state (a stepwise control system), or the boiler unit which starts lastly is controlled to shift alternately from the operating state to the halting state between the maximum and minimum output values (a continuous control system). In either case, the specific boiler unit is forced to frequently start and stop. As discussed above, the starting from the halting state always involves the above-mentioned prepurge and subsequent low output state. This is one of the factors which outstandingly degrade a response of the boiler system to the fluctuations in load.